1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to enhancements in cutting element placement so as to decrease the likelihood of bit tracking.
2. Background of the Technology
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface, actuation of downhole motors or turbines, or both. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole thus created will have a diameter generally equal to the diameter or “gage” of the drill bit.
An earth-boring bit in common use today includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones or rolling cone cutters. The borehole is formed as the action of the rotary cones remove chips of formation material that are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing a plurality of cutting elements on the cutters. Cutting elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits or “insert” bits, while those having teeth formed from the cone material are known as “steel tooth bits.” In each instance, the cutting elements on the rotating cutters break up the formation to form the new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (“ROP”), as well as its durability. The form and positioning of the cutting elements upon the cone cutters greatly impact bit durability and ROP, and thus are critical to the success of a particular bit design.
To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, decreased ROP, increased loading on the other cutting elements on the bit, and may accelerate wear of the cutter bearings, and ultimately lead to bit failure.
Conventional bits also typically include one or more rows of gage cutting elements. Gage cutting elements are mounted adjacent to the heel surface but orientated and sized in such a manner so as to cut the corner of the borehole. In this orientation, the gage cutting elements generally are required to cut both the borehole bottom and sidewall. The lower surface of the gage cutting elements engages the borehole bottom, while the radially outermost surface scrapes the sidewall of the borehole.
Conventional bits also include a number of additional rows of cutting elements that are located on the cones in rows disposed radially inward from the gage row. These cutting elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutting elements and, as used herein, may be described as bottomhole cutting elements. Such cutters are intended to penetrate and remove formation material by gouging and fracturing formation material. In many applications, inner row cutting elements are relatively longer and sharper than those typically employed in the gage row or the heel row where the inserts ream the sidewall of the borehole via a scraping or shearing action.
Increasing ROP while simultaneously increasing the service life of the drill bit will decrease drilling time and allow valuable oil and gas to be recovered more economically. Accordingly, cutting element placement for the rotatable cutters of a drill bit which enable increased ROP and longer bit life would be particularly desirable.